Rangefinder effective lengths - compared
Here is a table of the effective rangefinder baselengths (ERB) of most popular RF camera bodies:
|Leica M3||62 mm|
|Leica M 0.85||59 mm|
|Cosina Bessa T||58 mm|
|Leica II b-f & III b-g||58 mm|
|Nikon SP||58 mm|
|Zeiss Ikon (2004)||55.9 mm|
|Leica M 0.72||49 mm|
|Leica M8 0.68||47.1 mm|
|Konica Hexar RF||41 mm|
|Leica II/a & III/a||41 mm|
|Leica M 0.58||40 mm|
|Minolta CLE||28 mm|
|Cosina Bessa R||24 mm|
|Leica CL||18 mm|
So what is the "effective rangefinder baselength" anyway? It is the product of multiplying the physical rangefinder window separation distance by the viewfinder magnification. Why does it matter? Simply because the longer the effective RF baselength, the more accurate the focus for tele lenses and / or when shooting at wide-open apertures.
Applying the data from the above table, if you want to shoot a Noctilux 50mm lens wide-open at ƒ1.0, then a properly calibrated M3 will give you the most accurate focus. OTOH, mount the Noctilux on a Leica CL, and you may as well focus manually by looking at the lens barrel distance scale.
Erwin Puts has put together a technical page featuring a detailed explanation of the theoretical and practical limits of rangefinder focus accuracy. If you are into this sort of thing, wade through the following URL:
Finally, in Sep 2001 Leica announced the Leica Viewfinder Magnifier M 1.25x (12004). This is a small optical telescope which screws into the M eyepiece to increase the viewfinder magnification and so increase even more the ERB and focus accuracy.
According to their web site, the three current TTL M rangefinders receive the following improvements:
|Camera||New Mag.||ERB (mm)|
|Leica M 0.58||0.72||50.21 mm|
|Leica M 0.72||0.90||62.33 mm|
|Leica M 0.85||1.06||73.58 mm|
Thus a 0.85 M with the 1.25x eyepiece will give you a larger than lifesize image with a ERB of 73mm! There is a trade-off however, at this kind of magnification you will have trouble seeing the full 50mm frameset. Indeed Leica themselves only recommend using the 1.25x eyepiece to enhance focus accuracy and viewing with longer lenses (say the 90mm or 135mm).